Many industries utilize heavy metals and/or rare earth metals in their manufacturing processes. Such use typically results in liquid (generally aqueous) waste streams that contain residues of the rare earth or heavy metals utilized in the given manufacturing process. For example, the waste streams resulting from electronics, electroplating, and photographic processes typically contain metal ions such as copper, nickel, zinc, chromium (III), chromium (VI), cadmium, aluminum, lead, antimony, silver and gold, amongst others in various aqueous solutions such as sulfates, chlorides, fluoroborates and cyanides. Because of the potential adverse effect of such substances on health and the environment, the removal of rare earth metals and heavy metal ions from aqueous waste streams is a problem of continuing significance.
For the purposes of the present invention, heavy metals will be defined generally as elements having atomic numbers greater than 20, as defined by the Periodic Chart of the Elements and are metallic at ambient conditions. Rare earth metals are defined as those heavy metals having atomic numbers 57 through 71 inclusive. Actinides are those heavy metals having atomic numbers greater than 89. For example, aluminum, arsenic, antimony, copper, nickel, zinc, chromium, cadmium, mercury, platinum, palladium and gold are all heavy metals typically found in the waste stream of common manufacturing processes. In addition, cesium and strontium (and other radioactive metals) are found in aqueous waste streams in the nuclear industry.
The conventional and predominant method of treatment of the aqueous streams described above is the precipitation of the metal ions in the form of hydroxides or carbonates. That method of removing heavy metals is largely undesirable because it results in a sludge that is difficult and/or expensive to remove and separate from the waste stream. Furthermore, the recovered sludge is typically deposited in a hazardous waste site, raising additional environmental concerns. Finally, it is difficult to separate the individual metal from the resultant sludge for recycling back into the manufacturing process. Other recovery methods include evaporation, reverse osmosis, ion exchange, electrolytic metal recovery, and solvent extraction. These methods, however, have varying levels of success and do not typically allow for the quick and inexpensive separation and removal of the individual metals from the waste streams.
Another common technique for the separation and recovery of rare earths is solvent extraction. However, the main difficulty in a solvent extraction recovery process arises from the low concentration at which these metal ions exist in the aqueous stream generated from hydrometallurgical processes. Also, the identical complexing behavior of all the rare earths due to their similar ionic sizes and chemistry limits the ability to separate out the individual rare earth metals from the sample collected in the solvent extraction process. See Nakamura, Tachimori and Sato, 15 Journal of Nuclear Science and Technology, 829-834 (1978).
A more recent method of removing metals from waste streams features using compositions which include chelating agents that are bound to inorganic carriers. Chelating agents, also known as multidentate ligands, are compounds which are capable of complexing with various metal ions in solution where one chelation molecule has the capacity to attach a metal ion at two or more positions. Those chelating agents are molecules which contain one or more of the same donor atom (e.g., "electron sufficient" atoms such as oxygen, nitrogen, sulfur etc.) or two or more different donor atoms through which coordinate and/or covalent bonds are formed between the metal ion and chelating agent. One such composition is disclosed in U.S. Pat. No. 3,886,080 to Schucker et al. ("Schucker"). Schucker discloses a composition in which a specific type of chelating agent (i.e. oxine, dithizone, neocuproine O-phenanthroline, or salicylaldoxine) has been rendered immobile or insoluble by bonding the chelating agent to a silane coupling agent using a diazo linkage (i.e. .gamma.-aminopropyltriethoxysilane). The silane coupling agent, in turn, is bonded to the inorganic carrier.
The compositions defined by Schucker have many disadvantages. Initially, the method of making the compositions disclosed by Schucker inherently limits the types of compounds that can be utilized in the composition. For example, the only chelating agents that can be used are certain specific compounds having an unsaturated ring structure. Furthermore, because the chelate and the silane coupling agent are bound by a diazo linkage, only compounds capable of forming such a linkage can be used to produce the composition. Furthermore, the Schucker process for making the compositions is a five step process. The large number of steps required can result in decreased capacities (i.e., the amount of metal the composition is capable of chelating) due to the aggregate inefficiency of the chemical reactions utilized to produce a given composition. Lastly, the Schucker compositions are not capable of separating individual metals and, therefore, are not useful in metal recovery processes which seek to recycle individual metals back into the manufacturing process from which they came. Accordingly, there is a need for more cost efficient processes for the separation and removal of heavy metal and/or rare earth metals from waste streams by producing compositions having a variety of chelating agents which are specific and selective toward desired metal ions.